Wind turbines can be erected on-shore or off-shore. Large wind turbines being capable of providing an electric power up to about 6 MW are typically installed off-shore. In particular for maintenance reasons self-excited generators having a rotor assembly with permanent magnets are employed. In the near future wind turbines being capable of providing an electric power in the order of 15 MW will be provided by wind turbine manufactures having a special expertise in the field of off-shore wind turbines. For several technical reasons an electric generator being capable of providing 15 MW of electrical power must have a diameter in the order of 10 m. For efficiency reasons an air gap between (a) the coils of stator segments of the stator assembly and (b) the (permanent) magnets of the rotor assembly should be kept small. Therefore, such a large generator must be built up with an extreme high constructional precision. Further, the large size of such a generator requires special solutions not only for assembling but also for maintaining the generator. A maintenance work may include service tasks such as e.g. an exchange of a magnet on the rotor side and an exchange of a stator segment on the stator side.
The requested extreme high constructional precision requires a mechanically stable support or frame structure both for the rotor assembly and for the stator assembly. On the other side it is desirable to keep the weight (and the cost) of these assemblies within acceptable limits in order to allow for a safe handling when assembling such a large electric generator and when mounting the generator into a nacelle of a wind turbine, in particular into an off-shore wind turbine. In this respect it is further of importance to construct a stator assembly in such a manner that a cooling of components of the stator assembly, in particular of the stator segments, is possible.
EP 2 351 191 B1 discloses a stator assembly for a generator of a wind turbine. The disclosed stator assembly comprises a frame structure which has openings which for cooling purposes allow a stream of air passing through the frame structure.